Level shifter circuits are frequently used to translate one voltage level to another. For example, to conserve power, the "core" voltage within an IC frequently operates at low voltage, but is scaled up at the output. A special class of level shifters is the "half bridge drivers" in which complementary signals are applied to an "upper" and "lower" device. In this application, typically the lower device does not require any special signal processing. The upper device requires level shifting in order to provide adequate Gate-Source enhancement, which is necessary because the source potential is not referenced to ground.
FIG. 1 illustrates a typical half-bridge circuit with a level shifter. Level shifter circuit 10 comprises a first inverter (V1) 16, a pair of control devices 20 and 28 (N1 and N2, respectively), and a cross-coupled pair devices 18 and 24 (P1 and P2, respectively). Complementary signals are applied to N1 and N2 which in combination with P1 and P2 allow node 26 (Q') to swing between Vboost 11 and ground. A half-bridge circuit 108 comprising transistors 34 and 38 (N3 and N4, respectively) and a boost circuit is generated by diode 32(D1) in combination with "flying capacitor" 30 (C1). A major disadvantage of level shifter circuit 10 is that devices N1, N2, P1 and P2 are all vulnerable to the full voltage excursion from ground to Vboost. This requires special high voltage processing to minimize or avoid the potential of damages to these devices, which adds to manufacturing cost. Furthermore, since the gates of P1 and P2 are driven to twice the voltage as N1 and N2, toggling of these devices from one state to another are further aggravated and made more difficult. There is therefore a need for a level shifter circuit that provides the necessary voltage shifting while minimizing the vulnerability of the level shifter to voltage excursions that may damage the circuit devices, as well as improving performance and minimizing difficulties in toggling devices from one state to another.